FIG. 2 shows a schematic top view of a conventional micromechanical structure for an acceleration sensor, similar to the one described in German Published Patent Application No. 198 08 549. Such a structure may be, for example, used as a sensing element of a speed sensor.
In FIG. 2, 5′ indicates a centrifugal mass which has a conical oblong shape having a longitudinal axis L′. In this case, M′ is the longitudinal center point. Centrifugal mass 5′ is elastically suspended over substrate 1 via an anchoring device (not shown) and flexible springs 4a′ to d′, so that centrifugal mass 5′ is elastically deflectable from its rest position, i.e., it is a linear oscillator. 10a′–f′ are electrodes which are movable together with centrifugal mass 5′ and 100 is an electrode fixed rigidly in substrate 1. Each movable electrode 10a′–f′ is assigned such a rigidly fixed electrode, however, only fixed electrode 100 is shown for the sake of simplicity.
Such micromechanical acceleration sensors may be configured as spring-mass systems. In general, centrifugal mass 5′, springs 4a′ to d′, and electrodes 10a′ to f′ may be made of epitaxial polysilicon, which is made freely suspended by removing a sacrificial layer made of SiO2 between the epitaxial polysilicon and the substrate.
The acceleration is detected capacitively via an appropriate capacitor, which has movable electrodes 10a′ to f′ as capacitor plates, which are attached to centrifugal mass 5′, as well as fixed electrodes 100, which are anchored on substrate 1. Electrodes 10a′ to f′ attached to centrifugal mass 5′ may have stepped lengths having different natural frequencies. The reason for this is that the natural frequencies of electrodes 10a′ to f′ are not to interfere with the response of the acceleration sensor.
Conventional shapes of centrifugal mass 5′ are rectangles and the conical shape shown in FIG. 2. In the conventional approach shown in FIG. 2, the robustness of centrifugal mass 5′ may be too low in regard to interfering influences acting orthogonally to longitudinal axis L′.